1. Field of the Invention
The present disclosure relates to a scanning electron microscope (SEM), and more specifically, to a method and apparatus for monitoring electron beam condition of the SEM.
2. Description of the Related Art
SEM is widely used in various fields. For example, SEM can be used to detect wafer defects and measure critical dimensions in semiconductor manufacturing processing. Advantages of SEM include simple sample preparation, wide range of adjustable magnification, high image resolution, and large depth of field.
SEM operates using an electron beam to scan a surface of a sample in order to obtain information about the sample. The information may include surface structure, as well as physical and chemical properties of the sample. The SEM may include an electron beam system for producing the electron beam to scan the surface of the sample. The electron beam system may include an electron gun and an electromagnetic lens system. The electron gun is used to generate electrons. The electromagnetic lens systems may include a series of electromagnetic lenses for converging electrons emitted from the electron gun into an electron beam, and for focusing the electron beam onto the surface of the sample. Generally, it is preferable that the cross-section of the electron beam be relatively small and oval, so as to obtain a high image resolution during the scan.
In practice, the condition of the electron beam incident on the surface of the sample (hereinafter referred to “electron beam condition”) may be affected by various factors, such as drift in the positions of the electron gun or the lenses in the electromagnetic lens system, instability in the operating voltage of the SEM, or mechanical positioning of the sample stage. As a result of one or more of the above factors, the electron beam condition may become unstable and may vary over time, thereby impacting SEM imaging quality. For simplicity, when “electron beam condition” is referenced hereinafter, it refers to the condition of the electron beam incident on the sample surface. The above problem is not uncommon in SEM imaging applications for advanced semiconductor technologies, since resolution of the decreased node dimensions and feature sizes is highly dependent on the electron beam condition.
In order to maintain the electron beam condition, offline calibrations may be performed periodically on the SEM at a fixed schedule, for example, after every 10 to 12 hours of SEM operation. When performing an offline calibration, the SEM is calibrated by measuring a standard sample with known parameters, and adjusting the settings in the SEM based on the known parameters so as to maintain the electron beam condition.
Typically, offline calibrations of the SEM are performed periodically independent of the electron beam condition. In other words, offline calibrations of the SEM may be performed even if there has been no deterioration in the electron beam condition and no calibration is necessary. As a result, periodic offline calibrations can lead to unnecessary and additional down-time of the SEM, since the SEM has to switch from operation (or production) mode to an offline state during offline calibrations.
In addition, periodic offline calibrations do not allow the electron beam condition of the SEM to be monitored continuously. As a result, offline calibrations alone may be insufficient to maintain the electron beam condition at all times during operation of the SEM. For example, the electron beam condition may deteriorate during the time interval between offline calibrations (while the SEM is still operating), and the problem may not be detected or corrected until the next offline calibration is performed.
To mitigate the above problem, the time interval between offline calibrations may be reduced. However, this will increase the frequency of offline calibrations (and corresponding SEM down-time), which may disrupt SEM operation and result in a loss in SEM scanning productivity. Furthermore, offline calibrations may yield a slightly different electron beam condition after each offline calibration, and the slight differences in electron beam condition may lead to inconsistencies in the results of sample measurements.